The present invention relates to electrical measurement devices and, more particularly, to devices for measuring current.
Electric watthour meters, for example, employ load current applied to a current stator as part of the driving apparatus for rotating a metallic disk against the retarding force of a permanent magnet. Such devices are capable of direct measurement of energy consumption in the presence of currents as high as a few hundred amperes. For higher values of current, a current transformer may be provided to reduce the load current by a predetermined ratio. Such current transformers have a small number of primary turns and a large number of secondary turns. The output current in the secondary is approximately equal to the primary current divided by the turns ratio.
Problems occur when it is desired to produce a signal in the range of a few milliamperes in response to load currents in the range of several amperes to several hundred amperes. If, for example, a desired full-range output signal of 10 milliamperes is desired in response to a load current of 200 amperes, a ratio of about 20,000 is required. To attain such a ratio in a current transformer with a one-turn primary requires 20,000 turns on the secondary. It is difficult and expensive to wind this many turns on even a large transformer core, and the resistance of the resulting winding would be so high that poor performance of the transformer would result. When it is desired to perform such current scaling within reasonable size and cost constraints, techniques other than current transformers must be considered.
One prior art technique disclosed, for example, in U.S. Pat. No. 4,182,982 employs a current-carrying conductive plate with a window cut into it, thus dividing the plate into a resistive current divider in which a shunt resistance carries most of the current and a parallel measurement branch carries a fraction of the total current. A core having many turns is disposed on the measurement branch to produce an output current equal to the product of the resistive reduction and the turns ratio. This device is conventionally formed using a metal of high conductivity such as, for example, copper. Unfortunately, copper, as well as most other practical metals, has a high temperature coefficient of resistance. A slight temperature difference between the main current path and the measurement path is capable of producing an error sufficiently large to destroy the value of the current measurement for critical applications. In addition, the disclosed arrangement is prone to disturbance by magnetic fields produced by the current being measured and by external magnetic fields.
One attempt to solve the problem of external magnetic fields and temperature coefficient of resistance in copper is disclosed in U.S. Pat. No. 4,492,919 wherein two windows cut into a conductive plate produce a central conductive measurement bar of relatively large resistance flanked by symmetrically disposed shunting resistors. Although an improvement on the previously described device, this technique remains subject to disturbance by external magnetic fields and to measurement errors from unequal heating of the parallel paths.
U.S. Pat. Nos. 4,513,273 and 4,496,932 attempt to overcome problems with the temperature coefficient of resistance of copper by maintaining close thermal coupling between parallel current-carrying plates. Such devices lack the precision over the extreme current ranges with which the present invention must deal.
Certain metal alloys have been developed specifically for low temperature coefficient of electrical resistance. Materials having temperature coefficients of resistance of from about 5 to about 40 parts per million per degree centigrade are available at the time of filing of the present disclosure. Such materials are sold under trademarks such as, Nikrothal LX, Cuprothal 294, Karma, Advance, and Manganin. All of these alloys share the property that, besides a low temperature coefficient of resistance, they exhibit high resistivity and are expensive.